Method for determining COP generation factors for single-crystal silicon wafer

ABSTRACT

A whole determination area of a targeted wafer is concentrically divided in a radial direction, COP density is obtained in each divided determination segment, a maximum value of the COP density is set as COP density RADIUSMAX , a minimum value of the COP density is set as COP density RADIUSMIN , a value computed by “(COP density RADIUSMAX −COP density RADIUSMIN /COP density RADIUSMAX ” is compared to a predetermined set value, and a non-crystal-induced COP and a crystal-induced COP are distinguished from each other based on a clear criterion, thereby determining the COP generation factor. Therefore, a rejected wafer in which a determination of the crystal-induced COP is made despite being the non-crystal-induced COP can be relieved, so that a wafer production yield can be enhanced.

This application is a Divisional of U.S. Ser. No. 13/660,299 filed onOct. 25, 2012, which is a Divisional of U.S. Ser. No. 13/252,311 filedon Oct. 4, 2011, which received a U.S. Pat. No. 8,316,727 issued on Nov.27, 2012, which is a Divisional of U.S. Ser. No. 12/308,058 filed onDec. 5, 2008, which received a U.S. Pat. No. 8,061,225 issued on Nov.22, 2011, which is a national phase of PCT/JP2007/060438 filed on May22, 2007, which is published as WO/2007/142026 on Dec. 13, 2007.

TECHNICAL FIELD

The present invention relates to a method for determining COP (CrystalOriginated Particle) generation factors aimed at a single-crystalsilicon wafer.

BACKGROUND ART

A single-crystal silicon wafer as a semiconductor device substrate iscut out from a single-crystal silicon ingot, and is produced by beingsubjected to various physical, chemical, and thermal treatments. Thesingle-crystal silicon ingot is usually obtained by the Czochralskimethod (hereinafter referred to as “CZ method”) in which a seed crystalis dipped into silicon melted in a quartz crucible and pulled up to growa single crystal. However, micro defects called Grown-in defects areinduced in the crystal during the single crystal growth.

The Grown-in defects depend on a pulling-up speed during the singlecrystal growth and a temperature distribution (temperature gradient incrystal in a pulling-up axis-wise direction) in the single crystalimmediately after solidification. In the single crystal, the Grown-indefects appear in the form of a hole aggregation defect called COP(Crystal Originated Particles), having sizes ranging from about 0.1 to0.2 μm, or in the form of a defect including a micro dislocation calleda dislocation cluster, being about 10 μm in size.

In the single-crystal silicon wafer produced by the CZ method, anOxidation-induced Stacking Fault (hereinafter referred to as “OSF”)appearing in a ring shape may be generated when the single-crystalsilicon wafer is subjected to a high-temperature oxidation heattreatment. A potential area where the OSF ring is generated depends on athermal history of the crystal during growth, particularly influenced bya pulling-up speed during growth. The region where the OSF ring appearsgets shrunk from the outer circumferential side to the inner side of thecrystal as the pulling-up speed is lowered.

In other words, the inner side area of the OSF ring spreads to the wholearea of wafer when the single crystal is grown at a higher speed, andthe outer side area of the OSF ring spreads to the whole area of waferwhen the single crystal is grown at a lower speed.

In the case where OSF exists on a wafer surface which is of a deviceactivation area, the OSF causes a leak current to deteriorate a devicecharacteristic. COP is a factor which lowers an initial oxide-filmwithstand voltage, and the dislocation cluster also causes a defectivecharacteristic of the device formed therein.

Therefore, the single crystal is conventionally grown at a highpulling-up speed such that the ring-shaped OSF generation region islocated in the outer circumferential portion of the crystal. Forexample, as described in Japanese Patent Application Publication No.2002-145698, there is proposed a wafer in which the OSF region is widelydistributed from a circumferential edge portion to a central portion ofthe wafer and a micro COP region is formed inside the OSF region.

However, a single-crystal silicon wafer (hereinafter referred to as“defect-free crystal silicon wafer”) in which the number of Grown-indefects including extremely small COPs is decreased as much as possibleis produced with the advance of the fine process of the semiconductordevice to cope with growing demand on miniaturization and highperformance.

Accordingly, a COP evaluation is made in the defect-free crystal siliconwafer to make an acceptance determination in which crystal integrity(defect-free) is guaranteed by the number of defects (COPs) and thepresence or absence of a specific pattern through the COP evaluation. Inthe COP evaluation, a method called a copper deposition method (copperdecoration method) can be employed as an example of the method fordetecting COPs.

In the copper deposition method, an uneven insulating film (oxide film)is utilized in a region where the defects (COPs) exist when the oxidefilm is formed on a wafer surface. After the oxide film having apredetermined thickness is formed on the wafer surface, an externalvoltage is applied, copper is deposited while the oxide film isdestroyed in a defect region on the wafer surface, and the depositedcopper is observed to detect the defects (COPs) by the naked eye or witha Transmission Electron Microscope (TEM) or Scanning Electron Microscope(SEM).

The COP generation factors can be classified into the crystal-inducedand the non-crystal-induced. The crystal-induced COPs mean the Grown-indefects that are induced into the crystal during the single crystalgrowth.

According to the investigation thus far, it is found that the generationpatterns of the crystal-induced COPs are classified into the followingfour segments:

(1) The crystal-induced COPs appear in a disc shape in a central part ofwafer.

(2) The crystal-induced COPs appear in a ring shape in an peripheralpart of wafer as following its circumference.

(3) The patterns (1) and (2) simultaneously appear, namely, appearing ina disc-ring shape.

(4) The crystal-induced COPs densely appear in the whole surface of thewafer (300 counts or more in a wafer of 300 mm in diameter).

On the other hand, the non-crystal-induced COP is not COP in a strictsense, and caused by a micro flaw or scratch generated on the wafersurface during handling the silicon wafer. When the non-crystal-inducedCOPs are observed with a surface defect inspection apparatus (forexample, SP2: product of KLA-Tencor) or by the copper deposition method(copper decoration method), the COPs are generated in a line shape, orin a dot shape locally or in the whole surface of the wafer.

Since the non-crystal-induced COP is not an intrinsic defect derivedfrom the silicon single crystal itself, which is a constituent of awafer, the non-crystal-induced COPs should be removed from the COPevaluation object. In the current COP evaluation, COPs which are easilydetermined as the non-crystal-induced are removed from the evaluation.

However, there is no proper method for distinguishing thenon-crystal-induced COP and the crystal-induced COP from each other(that is, both are identified to thereby allow the non-crystal-inducedCOPs to be excluded in the evaluation), in particular, there is nomethod for determining that the non-crystal-induced COPs appearing in adot shape in the whole surface of the wafer are irrelevant tocrystal-induced COPs.

DISCLOSURE OF THE INVENTION

As described above, the COP evaluation has been carried out in thedefect-free crystal silicon wafer. However, unfortunately thenon-crystal-induced COPs which should be ruled out in the COPevaluation, particularly plain COPs appearing in a dot shape in thewhole surface of the wafer are hardly distinguished from thecrystal-induced COPs.

That is, determining plain COPs generated in a line shape or plain COPslocally generated in a dot shape as being non-crystal-induced is easilymade. However, in the case where cover-whole-surface COPs as beinggenerated in a dot shape in the whole surface of the wafer, determiningwhether the COPs are crystal-induced or non-crystal-induced is reallydifficult, while it is just left to an operator who is technicallyeducated how to determine. Even if COPs are generated, the wafer shouldbe acceptable so long as the COPs appear as being non-crystal-induced.Therefore, when such acceptable wafers should be rejected as beingnon-acceptable, a wafer production yield is unduly lowered.

Clarifying a criterion for a generation factor of such COPs appearing ina dot shape in the whole surface of the wafer can be cited as one ofmeans for enhancing accuracy of the acceptance determination. However,since the determination depends on the judgment by human eyes (visualinspection), it is difficult to delineate the criterion in words.

In the event when a production amount may increase from now, in orderthat the supply of the stable-quality wafer is realized while theproduction yield is enhanced, it is necessary to clarify the criterionfor the non-crystal-induced COPs, in other words, to quantitativelydefine the criterion by a numerical value. The clear criterion is alsonecessary to automate the COP evaluation (inspection).

In view of the foregoing, an object of the present invention is toprovide a quantitative method for determining COP generation factors fora single-crystal silicon wafer, in which the non-crystal-induced COP andthe crystal-induced COP are distinguished from each other based on aclear criterion in order to properly make the COP evaluation.

The present inventor makes investigations in order to solve the aboveproblem to establish the method capable of identifying COP generationfactors based on the clear criterion. As a result, the present inventordevises a method, in which the feature of the after-mentionedcrystal-induced COP generation behavior is utilized to check a densitydistribution obtained by computing the COP density in an r-direction(radial direction) or a θ-direction (circumferential direction) of thewafer against a threshold value set by a semi-empirical technique, or toconsider a COP generation position, thereby determining whether COPs areeither crystal-induced or non-crystal-induced.

The summary of the present invention includes the following methods (1)to (4) for making a COP evaluation on a single-crystal silicon wafer.

(1) A method for determining COP generation factors for a single-crystalsilicon wafer, in which a determination area of the wafer isconcentrically divided into segments in a radial direction, COP densityis obtained by enumerating in each divided determination segment, amaximum value of the COP density is set as COP density_(RADIUSMAX), aminimum value of the COP density is set as COP density_(RADIUSMIN), anddetermination that the COP generation factor is the one other than adefect induced during crystal growth is made when a value computed by“(COP density_(RADIUSMAX)−COP density_(RADIUSMIN))/COPdensity_(RADIUSMAX)” is not more than a predetermined set value.

(2) A method for determining COP generation factors for a single-crystalsilicon wafer, in which a determination area of the wafer isconcentrically divided into segments in a radial direction, anddetermination that COPs generated in segments other than a centralsegment and an outer circumferential segment among divided determinationsegments are attributed to a factor other than a defect induced duringcrystal growth is made.

In the methods for determining the COP generation factors for thesingle-crystal silicon wafer according to the above (1) or (2), a widthof each concentrically-divided determination segment desirably rangesfrom 15 mm to 30 mm.

(3) A method for determining COP generation factors for a single-crystalsilicon wafer, in which a determination area of the wafer isconcentrically divided in a radial direction and furthercircumferentially divided into segments, COP density of eachconcentrically-circumferentially-divided determination segment withinthe area of a concentrically-divided primary segment is obtained andassessed on the same radius basis, wherein a maximum value amongdetermination segments for each circumferentially-divided primarysegment is set as COP density_(CIRCUMFERENCEMAX), likewise a minimumvalue as COP density_(CIRCUMFERENCEMIN), and determination is made suchthat the COP generation is attributed to a factor other than a defectinduced during crystal growth, when a value computed by“(COPdensity_(CIRCUMFERENCEMAX)−COPdensity_(CIRCUMFERENCEMIN))/COPdensity_(CIRCUMFERENCEMAX)”is not less than a predetermined set value.

In the method for determining COP generation factors for asingle-crystal silicon wafer according to the above (3), a width of eachconcentrically-divided segment desirably ranges from 15 mm to 30 mm, andeach concentrically-divided primary segment is further circumferentiallydivided into three to eight determination segments.

(4) A method for determining COP generation factors for a single-crystalsilicon wafer, in which determination is made such that the COPgeneration is attributed to a factor other than a defect induced duringcrystal growth, when COPs are generated in a line shape, in adotted-line shape or in a dot shape locally.

In the methods for determining COP generation factors for asingle-crystal silicon wafer according to the above (1) to (4), the“single-crystal silicon wafer” is mainly directed to a silicon wafer of300 mm in diameter. That is, the evaluation method according to thepresent invention is mainly aimed at a large-diameter silicon waferhaving a diameter of 300 mm or more.

In the method for determining COP generation factors for asingle-crystal silicon wafer of the present invention, the determinationarea of the wafer is concentrically divided in a radial direction orfurther circumferentially divided, and the non-crystal-induced COP andthe crystal-induced COP are distinguished from each other based on COPdensity obtained in each determination divided segment or inconsideration of COP generation position. In the determination method ofthe present invention, the determination of the COP generation factorcan be made based on the clear criterion.

According to the method of the present invention, the rejected wafer inwhich the determination as the crystal-induced COP had been madeirrespective of non-crystal-induced can be relieved/rescued (accepted)to thereby enhance the wafer production yield. The clear criterion canprovide the stable-quality wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing an example of a Grown-in defectdistribution state.

FIGS. 2( a) and 2(b) are views showing COPs in samples wheredetermination as non-crystal-induced COPs could not be made at anearlier stage of an investigation on a method for determining COPgeneration factors.

FIG. 3 is a chart showing a process of evaluating COPs on asingle-crystal silicon wafer.

FIG. 4 is a chart showing a COP evaluation result of the single-crystalsilicon wafer in the COPs evaluation process.

BEST MODES FOR CARRYING OUT THE INVENTION

The methods for determining COP generation factors for a single-crystalsilicon wafer according to the above (1) to (4) of the invention will bespecifically described below. In the targeted COPs of the presentinvention, the number of COPs and their distribution are visuallyinspected on the wafer surface using a surface defect inspectionapparatus (for example, SP1: product of KLA-Tencor).

Generally, the crystal-induced COPs appear in a disc shape, ring shape,or disc-ring shape. The crystal-induced COPs may also appear in highdensity in the whole surface of the wafer. The reason why COPs appear insuch patterns is that the following particular relationship holdsbetween a pulling-up speed and a distribution of the Grown-in defectsinduced during the single-crystal growth when a seed crystal is dippedin silicon melt in a quartz crucible and pulled up.

FIG. 1 is a view schematically showing an example of a defectdistribution state of a section of a silicon single crystal pulled up bya growth apparatus which can produce a wafer having an extremely smallamount of Grown-in defects, while the defect distribution state iscontrasted with the pulling-up speed. FIG. 1 shows the observationresult of the micro-defect distribution state in which a grown singlecrystal is cut along a pulling-up axis, the cut single crystal is dippedin a copper nitrate solution to deposit Cu, and the micro defects areobserved by X-ray topography after heat treatment.

In FIG. 1, “V-rich” means an area where the number of COPs is increased,and “I-rich” means an area where a number of dislocation cluster defectscaused by trapped interstitial atoms during the single-crystal growthexist. “P-band” means an Oxygen-induced Stacking Fault (OSF). “PV”located on a lower pulling-up speed side of the “P-band” is an oxygenprecipitation promotion region which is of a defect-free region wherevacancies are dominant, and “PI” located on a further lower pulling-upspeed side of the “PV” is an oxygen precipitation suppression regionwhich is of a defect-free region where the interstitial atoms aredominant.

In the case where the single crystal is pulled up at a pulling-up speedcorresponding to the defect-free region, the wafer having an extremelysmall amount of Grown-in defects is obtained. However, in the case wherethe pulling-up speed is shifted toward the higher-speed side, COPshaving a specific pattern are generated. For example, in the case wherea cross section of the silicon single crystal has the defectdistribution state shown in FIG. 1, the wafer in which COPs in adisc-shaped pattern exist in the central portion is obtained when thesingle crystal is pulled up at a pulling-up speed Vd.

Depending on a thermal history in a pulling furnace, an in-planedistribution of the defects (LPD, OSF) on the V-rich side can have aconfiguration in which the central portion is stretched toward thelow-speed side or a configuration in which the outer circumferentialportion is stretched toward the low-speed side, in addition to theconfiguration as shown in FIG. 1 in which the central portion and theouter circumferential portion are substantially evenly stretched towardthe low-speed side. However, assuming that r designates a radius, aportion having about r/2 (that is, a distance of about ½ from the centerin a radial direction) cannot be stretched toward the low-speed side.Accordingly, the crystal-induced COPs should have a disc-shaped patternand/or a ring-shaped pattern. In particular, the ring-shaped pattern isnot generated at a position near r/2, but is generated in a manner tofollow the outer circumference of the crystal.

Since the thermal history is symmetric about the axis (pulling-up axis),COPs are evenly generated also in a θ-direction (circumferentialdirection of the wafer). Accordingly, density of COPs generated in aring shape or disc shape is substantially uniform in a circumferentialdirection.

COPs appear in dense in the whole surface of the wafer when thepulling-up speed is largely shifted toward the high-speed side. In sucha case, the COP density becomes high, and the number of COPs is not lessthan 300-400 in the whole surface of the wafer. Accordingly, even if theCOPs should appear in the whole surface of the wafer, when the number ofCOPs is not more than about 200, it is considered that the COPgeneration factor is not crystal-induced.

The crystal-induced COPs exhibit such a generation behavior as describedabove. The method for determining a COP generation factor according tothe present invention utilizes the generation behavior to determinewhether COPs are crystal-induced or non-crystal-induced.

The determination method (1) of the present invention is aimed at thesingle-crystal silicon wafer having extremely small amount of Grown-indefects. In the method (1), the determination area of the wafer isconcentrically divided into segments in a radial direction, the COPdensity is obtained in each divided determination segment, and thedetermination that the COP generation factor is attributed to a factorother than the defect induced during crystal growth is made when thevalue computed by “(COP density_(RADIUSMAX)−COP density_(RADIUSMIN))/COPdensity_(RADIUSMAX)” is not more than a predetermined set value. As usedherein, COP density_(RADIUSMAX) means a maximum value of the COP densityobtained in each divided determination segment, and COPdensity_(RADIUSMIN) does a minimum value likewise.

The determination method (1) is the one which focuses on a COP densitydistribution in an r-direction (radial direction of the wafer). Thevalue of “(COP density_(RADIUSMAX)−COP density_(RADIUSMIN))/COPdensity_(RADIUSMAX)” (hereinafter referred to as “evaluation function”)computed by obtaining the COP density in each divided determinationsegment is an index indicating whether or not the COP distribution iseven. When the evaluation function has a small value, it can be saidthat a difference in COP density between the divided determinationsegments is small and the COP distribution is even in an r-direction onthe wafer surface.

As described above, since the crystal-induced COPs have a disc-shapedpattern or a ring-shaped pattern, the COP distribution is not even in anr-direction and the evaluation function “(COP density_(RADIUSMAX)−COPdensity_(RADIUSMIN))/COP density_(RADIUSMAX)” has a large value when thecrystal-induced COPs are generated. Accordingly, when the value of theevaluation function “(COP density_(RADIUSMAX)−COPdensity_(RADIUSMIN))/COP density_(RADIUSMAX)” is not more than apredetermined set value, COPs can be regarded as not having adisc-shaped pattern and/or a ring-shaped pattern, and it can bedetermined that the COP generation is attributed to the factor otherthan the defect induced during the crystal growth; (that is,non-crystal-induced COPs).

The set value is determined in a semi-empirical fashion based on actualresults in the conventional determination.

In the determination method (2), the determination area of the wafer isconcentrically divided into segments in a radial direction, anddetermination is made such that COPs generated in segments other thanthe central segment and the outer circumferential segment among divideddetermination segments are attributed to the factor other than thedefect induced during the crystal growth.

The determination method (2) focuses on COP generation positions in ar-direction. The crystal-induced COPs have a disc-shaped pattern or aring-shaped pattern, and are not generated in segments other than thecentral and outer circumferential segments of the wafer, so that theCOPs generated in the segments other than the central and outercircumferential segments can be regarded as the non-crystal-inducedCOPs.

In the determination method (1) or (2), the width of eachconcentrically-divided determination segment (i.e., ring) desirablyranges from 15 mm to 30 mm.

The determination methods are mainly aimed at the wafer having adiameter of 300 mm. Generally, since the ring-shaped segment of theoutermost circumference having a width of 10 mm is ruled out fromevaluation, assuming that the wafer has a diameter of 300 mm, thedetermination segment pertinent to evaluation ranges from the center ofthe wafer to a radius of 140 mm. In the case where the area ranging fromthe center of the wafer to the radius of 140 mm is divided into ringshape segments, when the width of the ring is narrower than 15 mm, thenumber of determination segments is excessively increased to complicatethe evaluation, which leads to cost increase.

When the width of the ring segment is wider than 30 mm, the evaluationbecomes coarse to lose accuracy of the evaluation. Therefore, the widthof about 25 mm is suitable to the wafer having a diameter of 300 mm.Each determination segment is preferably evenly divided to have an evenwidth, but necessarily limited to it, and the width may be flexiblydefined as appropriate based on the COP generation condition and thelike.

In the determination method (3), the determination area of the wafer isconcentrically divided in a radial direction to primary segments andfurther circumferentially divided to determination segments, i.e.,relevant segments, the COP density of each concentricallycircumferentially divided determination segment, i.e., relevant segment,is obtained and assessed within each primary segment that is defined byconcentrically dividing the whole determination area of the wafer, amaximum value in a primary segment of the same radius is set asCOP-density_(CIRCUMFERENCEMAX), a minimum value is set as COPdensity_(CIRCUMFERENCEMIN), and determination is made such that the COPgeneration factor is attributed to the factor other than the defectinduced during the crystal growth, when the value computed by “(COPdensity_(CIRCUMFERENCEMAX)−COP density_(CIRCUMFERENCEMIN))/COPdensity_(CIRCUMFERENCEMAX)” is not less than a predetermined set value.As used herein, the COP density_(CIRCUMFERENCEMAX) means a maximum valueof the COP density obtained among relevant determination segments withina primary segment of the same radius, and the COPdensity_(CIRCUMFERENCEMIN) means a minimum value likewise.

The determination method (3) focuses on a COP distribution behavior in a9-direction (circumferential direction of the wafer). As describedabove, since the thermal history of the grown single crystal issymmetric about the axis of pulling-up, COPs are evenly generated in aθ-direction, and the density of COPs generated in a ring shape patternor disc shape pattern is substantially uniform in a θ-direction.

Accordingly, in the determination method (3), the whole determinationarea of the wafer is concentrically divided in a radial direction toprimary segments and further circumferentially divided to relevantdetermination segments, and the COP density of each relevantdetermination segment is obtained and assessed within each primarysegment of the same radius as defined above. When the value of theevaluation function“(COP density_(CIRCUMFERENCEMAX)−COPdensity_(CIRCUMFERENCEMIN))/COP density_(CIRCUMFERENCEMAX)” is not lessthan the predetermined set value, determination is made such that theCOP generation factor is non-crystal-induced since the COP density ofeach relevant determination segment in a circumferential direction isnot uniform.

In the determination method (3), a width of each concentrically-dividedprimary segment (i.e., ring) desirably ranges from 15 mm to 30 mm, andeach primary segment is circumferentially divided into three to eightrelevant determination segments.

The reason why the width of each concentrically-divided primary segment,a ring, ranges from 15 mm to 30 mm is already described as above. Thereason why each ring is circumferentially divided into three to eightrelevant determination segments is as follows. The determinationaccuracy is not enhanced too much even if the number of relevantdetermination segments divided in a circumferential direction isexcessively increased. When the ring is divided into two segments, sincethe relevant determination segment is excessively wide in a θ-direction,the change in COP density does not appear prominently even if the COPdistribution density varies in a θ-direction, which lowers thedetermination accuracy.

In the determination method (4), determination is made such that the COPgeneration factor is attributed to the factor other than the defectinduced during the crystal growth, when COPs are generated in a lineshape, in a dotted-line shape or in a dot shape locally.

COPs generated in a line shape, in a dotted-line shape or in a dot shapelocally are clearly attributed to the micro flaw and scratch generatedon the sample surface, so that these COPs can be easily determined asbeing non-crystal-induced. Accordingly, such COPs are ruled out fromdetermination.

The result of the determination, in which the COP generation factor isdetermined based on the determination method of the present invention toinvestigate whether or not the determination method can be applied, willbe described below.

The following methods (a) to (e) are used in the determination. Themethods (a) to (c) (a determination method (1)) focus on the COP densitydistribution in an r-direction of the wafer, the method (d) (adetermination method (2)) focus on the COP generation position in anr-direction, and the method (e) (a determination method (3)) focus onthe COP distribution behavior in a θ-direction of the wafer.

(a) The method in which the determination of the non-crystal-induced COPis made when the COP density distribution is uniform in the range of(φ50 to φ250

A ring-shaped area of φ50 to φ250 is divided into plural segments in anr-direction, the value of the evaluation function “(COPdensity_(RADIUSMAX)−COP density_(RADIUSMIN))/COP density_(RADIUSMAX)” isobtained in each determination segment, and the determination ofnon-crystal-induced COP is made when the value is not more than thepredetermined set value (threshold). The range of “φ50-φ250” indicates aring-shaped area between circles of 50 mm and 250 mm in diameter.

(b) The method in which the determination of the non-crystal-induced COPis made when the COP density distribution is uniform in the range ofφ100 to φ250

In the method (b), the COP observation position is changed from that ofthe method (a), and others are identical to those of the method (a).

(c) The method in which for a rejected wafer by a COP evaluation method,the determination whether or not COPs in the rejected area arenon-crystal-induced is made based on the COP density distribution.

The above COP evaluation method is used to make the acceptancedetermination whether or not the crystal integrity (defect-free) isguaranteed by the number of COPs and the presence or absence of aspecific pattern. In the COP evaluation method proposed herein by thepresent inventor, the whole determination area of the wafer isconcentrically divided in a radial direction to segments, an upper limitvalue of the number of COPs is set in each divided determinationsegment, and the acceptance determination is made based on the upperlimit value. This method is referred to as “new evaluation method”.

The determination segment where the rejection as non-acceptable is made(hereinafter referred to as “NG”), that is, the determination segmentswhere the number of COPs exceeds the reference value are extracted ineach rejected wafer by the “new evaluation method”, and the COP densityis computed. The value of the evaluation function “(COPdensity_(RADIUSMAX)−COP density_(RADIUSMIN))/COP density_(RADIUSMAX)” isobtained only for the extracted determination segments, and thedetermination of the non-crystal-induced COP is made when the value ofthe evaluation function is not more than the predetermined set value(threshold). However, in the case of only one segment where the numberof COPs exceeds the reference value, the COP density_(RADIUSMIN) is setto 0, and the value of evaluation function is set to 1.

(d) The method in which the determination whether or not COPs arenon-crystal-induced is made based on the COP generation position

It is assumed that COPs generated in the range of φ50 to φ200 are thenon-crystal-induced COPs.

(e) The method in which the determination of the non-crystal-induced COPis made, for the rejected wafer by the new evaluation method, when theCOP density distribution is not uniform in a θ-direction of the wafer

The wafer is divided into first to fourth quadrant, the COP density isobtained from the number of COPs counted in each quadrant, and thedetermination of the non-crystal-induced COP is made when the valuecomputed by the evaluation function “(COPdensity_(CIRCUMFERENCEMAX)−COPdensity_(CIRCUMFERENCEMIN))/COPdensity_(CIRCUMFERENCEMAX)”in the relevant determination segments within a primary segment of theidentical radius is not less than the predetermined set value(threshold).

Table 1 shows the determination result obtained by the methods (a) and(b). The determination of the non-crystal-induced dot-shaped COP is madeby the visual inspection for nine wafers (samples) used as thedetermination object (also shown in Table 1 as a reference). Thethreshold is changed in two levels for both the methods (a) and (b), andthe determination is made on four conditions in total.

TABLE 1 Determination method (a) (b) Determination Determination basedon COP based on COP Visual density density determination distributiondistribution (reference) in φ50-φ200 in φ100-φ250 Threshold — 0.5 0.70.5 0.7 The number of 9 7 7 7 8 determinations

As shown in Table 1, for the nine samples in which the determination ofthe non-crystal-induced dot-shaped COP is made by the visual inspection,the determination of the non-crystal-induced dot-shaped COP can be madefor seven samples by the determination method (a), and for seven oreight samples by the determination method (b).

FIGS. 2( a) and 2(b) are views showing COPs in the samples where thedetermination of the non-crystal-induced COP cannot be made by thedetermination method (a) or (b), in which FIG. 2( a) shows the samplewhere the determination of the non-crystal-induced COP cannot be made onthe four conditions in the determination methods (a) and (b), and FIG.2( b) shows the sample where the determination of thenon-crystal-induced COP can be made in the case where the threshold isset to 0.7 in the determination method (b) while the determination ofthe non-crystal-induced COP cannot be made in the remaining threeconditions. In FIG. 2( a), a portion surrounded by a broken-line ellipseindicates the linearly-generated non-crystal-induced COPs.

Table 2 shows the determination result obtained by the methods (c) and(d). The same nine samples as the determination method (a) or (b) areused in the methods (c) and (d).

TABLE 2 Determination method (c) (d) Determination Determination basedon COP based on COP density generation distribution in position in NG NGportion portion Threshold 0.8 — The number of 8 3 determinations

In the determination method (c), as described above, for the rejectedsamples by the new evaluation method, the value of the evaluationfunction “(COP density_(RADIUSMAX)−COP density_(RADIUSMIN))/COPdensity_(RADIUSMAX)” is obtained for only the segment where the numberof COPs exceeds the reference value to become NG, and the determinationof the non-crystal-induced is made when the value of the evaluationfunction is not more than 0.8. In the determination method (c), thedetermination of the non-crystal-induced can be made for the eightsamples of the nine non-crystal-induced samples.

The determination method (d) is one in which the determination of thenon-crystal-induced is made for COPs generated in the range of φ50 toφ200. In the determination method (d), the determination of thenon-crystal-induced can be made for the three samples of the ninesamples in which the non-crystal-induced COPs are generated.

The determination method (e) in which the determination of thenon-crystal-induced COP is made by the distribution behavior in theθ-direction is applied to the same nine samples as the determinationmethod (a) or (b). That is, each determination area divided in the ringshape is divided into the first to fourth quadrants, the evaluationfunction “(COP density_(CIRCUMFERENCEMAX)−COPdensity_(CIRCUMFERENCEMIN))/COP density_(CIRCUMFERENCEMAX)” is computedbetween the quadrants in the same primary, ring segment, the thresholdis set to 0.8, and the determination that COPs generated in the waferare non-crystal-induced is made when the relationship of “(COPdensity_(CIRCUMFERENCEMAX)−COP density_(CIRCUMFERENCEMIN))/COPdensity_(CIRCUMFERENCEMAX)”>0.8 holds in all the ring segments.

As a result, the determination of the non-crystal-induced COP can bemade for only one sample.

Accordingly, the determination method (e) is not suitable to the methodfor determining the non-crystal factor to the sample in which thenon-crystal-induced COPs are generated. Thus, an example application ofthe method for determining COP generation factors of the presentinvention is described.

The determination method (e), that is, the method described in (3) inwhich the determination of the non-crystal-induced COP is made based onthe distribution behavior in a θ-direction is not suitable to thedetermination method for the samples used in the example of application.However, as described below, the determination method (e) does not losesignificance as the method for determining COP generation factors of thepresent invention.

When the determination methods (1) to (3) are actually applied, it isnecessary that, in consideration of the COP generation (existence) statein the produced wafer and a quality level required for the wafer, thesemi-empirical technique of utilizing the actual results of the past COPgeneration factor determination be taken to define a specific criterionsuch as the width of the divided determination segment and a settingvalue (threshold).

Accordingly, the determination method (3) can be established as thesufficiently applicable determination method by finding the more propercondition for the specific criterion.

In applying the method for determining COP generation factors of thepresent invention to the COP evaluation of the single-crystal siliconwafer, obviously the method of the present invention can be solelyapplied, the determination of the COP generation factor is made based onthe clear criterion, and the non-crystal-induced COP can bedistinguished from the crystal-induced COP (ruled out from the COPevaluation). However, the determination method of the present inventioncan be more effectively utilized to enhance the COP evaluation accuracyand reliability of the single-crystal silicon wafer by a combinationwith the conventional COP evaluation method of the single-crystalsilicon wafer, desirably the improved COP evaluation method.

A system (process) for evaluating COPs of the single-crystal siliconwafer, in which the method for evaluating COP generation factors of thepresent invention is combined with the “new evaluation method”, will bedescribed below. The COP evaluation process is based on the result ofthe investigation whether or not the above-described determinationmethod of the present invention is applicable. For example, when thewafer to be evaluated is changed due to the change in productioncondition, since the COP generation (existence) state is changed in thewafer, more proper setting may be required for the width of the divideddetermination segment or the threshold.

In the process for evaluating COPs of the single-crystal silicon wafer,in which the method for determining COP generation factors of thepresent invention is incorporated, the procedure (process) is asfollows.

The procedure 1: The determination is made based on the number of COPsin the whole surface of the wafer. The determination is made for theacceptable wafer by the “new evaluation method” of procedure 2. Therejected wafer is transferred to the procedure 3.

The procedure 2: The acceptance determination of the silicon wafer ismade by the “new evaluation method”. The pattern determination is madein the acceptance determination of the procedure 2.

The procedure 3: For the rejected wafer in the procedures 1 and 2(particularly, the number of COPs exceeds the reference value, andtherefore the total number of COPs exceeds the reference value), basedon the COP density distribution, the determination whether or not COPsare non-crystal-induced is made. For example, the method (a) fordetermining COP generation factors is performed with the setting value(threshold) of 0.7. The final determination is visually made for theacceptable wafer in the procedure 3.

The procedure 4: For the rejected wafer in the procedure 3, based on theCOP generation position, the determination whether or not COPs arenon-crystal-induced is made. For example, the method (d) for determiningCOP generation factors is performed with the setting value (threshold)of 0.8. The final determination is visually made for the acceptablewafer in the procedure 4.

Since the initial application of the COP evaluation process is a testoperation, the visual determination is made in the procedures 3 and 4 toconfirm adequacy of the width of the divided determination segment ofthe wafer and the setting value (threshold).

FIG. 3 is a chart schematically showing a process of evaluating COPs ofthe single-crystal silicon wafer. The “procedure 1” to “procedure 4” andthe methods (a) to (d) of the procedures 3 and 4 in the FIG. 3correspond to the “procedure 1” to “procedure 4” and the methods (a) to(d) described above. The “acceptable” and “rejected” surrounded bysolid-line frames indicate acceptable and rejected in a mid-flow stageof the COP evaluation, and the “acceptable” and “rejected” surrounded bydouble solid lines indicate final result of the COP evaluation.

As shown in FIG. 3, when the method for determining a COP generationfactor of the present invention is combined with the “new evaluationmethod”, the rejected wafer in the COP total number determination(procedure 1) or the rejected wafer in the pattern determination(procedure 2) are relieved due to the determination of thenon-crystal-induced COP (see an arrow shown by a bold solid line), sothat the wafer production yield can be enhanced.

FIG. 4 is a chart showing a COP evaluation result of the single-crystalsilicon wafer performed by the above process of evaluating COPs. In FIG.4, the process of evaluating COPs is applied to the defect-free crystalsilicon wafer (n=173).

In the horizontal axis of FIG. 4, “>φ50” means a disc-shaped segmenthaving a diameter of lower than 50 mm, and “φ50-φ100” means aring-shaped segment having a diameter of 50 mm to 100 mm. In each of thedetermination segments from “>φ50” to “φ250-φ280”, the reject ratio is aratio of the rejected wafer based on each of the criteria shown in thelower portion of FIG. 4 to the total number of wafers, and the rejectratio in each determination segment includes the overlapping wafer.

The reject ratio in the “new evaluation method” of the horizontal axisindicates the total number (excluding the overlapping) of the rejectedwafers based on each of the criteria. The reject ratio in the“crystal-induced” is a reject ratio, in which the determination methodof the present invention incorporated in the COP evaluation process isapplied to distinguish the non-crystal-induced COP and thecrystal-induced COP from each other, and the wafer in which thedetermination of the non-crystal-induced COP is made is removed from therejected wafer the “new evaluation method” of the horizontal axis.

As can be seen from FIG. 4, when the determination method of the presentinvention is applied, the reject ratio is decreased from 0.139 to 0.087,thereby enhancing the wafer production yield.

INDUSTRIAL APPLICABILITY

In the method for determining COP generation factors for asingle-crystal silicon wafer according to the present invention, thenon-crystal-induced COP and the crystal-induced COP are distinguishedfrom each other based on the clear criterion to determine the COPgeneration factors. According to this method, the rejected wafer inwhich the determination of the crystal-induced COP is made despite beingnon-crystal-induced COP can be relieved, so that the wafer productionyield can be enhanced.

Accordingly, the method for determining a COP generation factor of thepresent invention can be suitably applied to the production of thesingle-crystal silicon wafer and the production of the semiconductordevice.

The invention claimed is:
 1. A method for determining a COP generationfactor of a single-crystal silicon wafer comprising: concentricallydividing a determination area of the single-crystal silicon wafer in aradial direction to form divided determination segments, determining forCOPs generated in the divided determination segments, except for acentral portion and an outer circumferential portion in the divideddetermination segments and for COPs generated in one or more of a lineshape, a dot-line shape, and a dot shape locally in all of the divideddetermination segments that the COP generation factor is attributed to afactor except for a defect induced during crystal growth.
 2. The methodfor determining a COP generation factor of a single-crystal siliconwafer of claim 1, wherein a width of each concentrically-divideddetermination segment ranges from 15 mm to 30 mm.